7 6 PLANT PHYSIOLOGY. 



The use of absorption of water by dry tissues to produce 

 movements is important and widespread in plants. 



37. What is the nature of the absorption in hygroscopic awns ? 

 Answer by Experiment 18. 



EXPERIMENT 18. Mount a hygroscopic awn of Erodium or Stipa 

 by fixing it vertically on a large cork with sealing-wax ; if the upper 

 end does not possess a horizontal projection usable as a pointer, 

 add one with sealing-wax. Place successively in wet and dry 

 chambers and observe movements of the pointer. Explain the 

 results upon a micellar basis. (Place it also in a steam-jet.) 

 A telling demonstration of the power with which dry tissues absorb 

 water is given by placing dried peas (Soja beans are particularly effec- 

 tive) in a narrow-necked bottle, until it is full, when it is to be filled also 

 with water and immersed in water. The process is, however, here com- 

 plicated by osmotic phenomena. 



The mechanics of the twisting of hygroscopic awns is discussed by 

 Murbach in Botanical Gazette, 30, 113. The great value of Soja beans 

 for imbibition experiments is pointed out by Copeland in the same 

 journal, 29, 347. 



38. What is the power which produces the ascent of sap ? 



No experiment upon this subject is practicable. Although 

 obviously of very great importance, it is still unsettled. A full 

 theoretical treatment of it should be worked out and a concise 

 account given. 



One phase of it which may easily be studied experimentally is the 

 influence of root-pressure in aiding the ascent in small herbaceous plants. 

 This may readily be investigated as follows : Root-pressure is to be 

 removed by cutting off the shoot and placing it in water ; then, after it 

 begins to wilt, the pressure is to be restored by placing the shoot in water 

 in one arm of a long U-tube and pouring mercury into the other arm. 

 (See MacDougal, Physiology, 32.) The mercury tube used for testing 

 joints (see page 41) is well adapted for this purpose. 



A number of practicable experiments upon this subject are given by 

 Darwin and Acton, 88-96. The experiments given in some works to 

 illustrate the lifting power of evaporation seem to me to have no bear- 

 ing upon this question, since the physical conditions, particularly the 

 operation of atmospheric pressure upon the exposed liquid, are so very- 

 different in the two cases. 



